When workpieces of different types are being machined, they have to be firmly held in a stationary manner in a specific position, or they have to be clamped to a movable or immovable machine part in a machining unit. In both cases, the clamping apparatus used for this not only has to position the workpiece but also has to absorb and transmit the forces arising from a machining operation. In the process, the shape of the workpiece must not be changed, either by the clamping forces or by the machining forces. This- applies, above all, to thin-walled workpieces. These forces are for the most part absorbed by a frictional connection.
Clamping sleeves are often used to clamp round workpieces to a machining unit. The clamping sleeves are designed to be relatively thin-walled. One side of the clamping sleeve serves as a contact surface on the workpiece, to be specific either on an external surface of the workpiece or on an internal surface of a hollow workpiece. On the other side, there is a relatively narrow annular space between the clamping sleeve and the machine part which is adjacent to it and in which the clamping sleeve is fitted. This annular space is sealed off in both axial directions. If a hydraulic or pneumatic operating medium in this annular space is pressurized, the clamping sleeve deforms elastically in the direction of the workpiece and clamps the latter firmly. Because the clamping sleeve, for its part, is connected to the machine part which accommodates it by a positive connection or a frictional connection, it is possible for the machining forces acting on the workpiece to be transmitted to the machine part, or conversely from the machine part to the workpiece.
The clamping sleeves have the disadvantage that their ability to be deformed radially is only very low and, in addition, this ability decreases further from the central longitudinal section outward to the two end sections. Clamping sleeves can therefore be used only to clamp workpieces which lie within a very small range of diameters. The consequence of this is that workpieces which are not machined in the clamping area often cannot be firmly clamped by means of clamping sleeves.
The chucks which are used for clamping workpieces with an adequately great dimensional strength and which generally have three clamping jaws cannot be used in the case of workpieces with a low dimensional strength, in particular in the case of thin-walled workpieces. The clamping tongues which are often used in addition cannot reliably clamp workpieces having large tolerances on the diameter or shape deviations, for example non-roundness. In addition, in the case of these chucks the radial clamping force is distributed to a few circumferential points, in particular when the actual diameter of the workpiece does not coincide exactly with the nominal diameter of the clamping tongues. Thin-walled workpieces are then subjected to an increased risk of deformation.